1 David P. Pappas National Institute of Standards and Technology Boulder, CO CNRS Thematic School High Sensitivity Magnetometers Sensors & Applications

2 Health care Non-invasive medical evaluation Magnetic liver & lung “biopsies” Magneto-cardiography (MCG) Magneto-encephalography (MEG) BARC Geophysical exploration Surveying Locating artifacts Non-destructive testing Bridges Electronics Current monitoring Data storage technology Millitary applications Mine detection Naval situations Applications of high sensitivity magnetic sensors => Magnetic sensors have large impact

3 F.L. Fagaly Magnetics Business & Technology Summer fT 1 nT  , Frequency (Hz) Magnetic field Range 1 pT Geophysical Industrial Magnetic Anomaly Magneto- cardiography Magneto- encephalography High sensitivity magnetometer applications field vs. frequency

4 High Sensitivity Magnetic Field Sensors Superconducting SQUID High, low T C Semiconductors Hall Magneto-electric EMR - nanostructures Resonance M Z - Protons M X - Electrons Ferromagnetic Fluxgate Induction coil Magneto-resistive (AMR,GMR, TMR) Giant Magneto-Impedence Magneto-optic Magneto-strictive

5 Sensor specifications –Equivalent noise spectral density referred to the input –Sensitivity (V/T, Hz/T, A/T…) –Dynamic range, linearity, slew-rate –Working type (flux locked vs. open loop) –Offet, temperature stability, Resistance against environment (humidity, vibration) –Hysterisis, Perming –Cross talk between field components – cross field –Bandwidth –Form factor – spatial resolution –Power –Price –Operating temperature (cryogenics) –Multi-sensors

6 Oh, et. al JKPS (2007) SQUID Noise Spectral Density – T/  Hz High T C SQUID Low T C SQUID Frequency (Hz) pT/  Hz 10 fT/  Hz Penna et. al Phil M B (2000) 100fT/  Hz Why is noise spectrum important? Integrated over bandwidth of the sampling! How to read these graphs: ultiply by the  frequency ~1 pT/  1Hz ~20 fT/  1Hz

7 Example – Magneto-cardiography Beats are not perfectly regular Heart signals edges go up to > 500 Hz Need bandwidth ~ 1 kHz for real-time and full signal analysis ~ 30x noise in raw LTC data Oh, et. al JKPS (2007)

8 Technique Noise floor (1 Hz) SQUID 1 femto-Tesla Optical pumping 500 femto-Tesla Fluxgates 1 pT /  Hz Magneto-resistive 100 pT/  Hz at 1 Hz Hall effect 100 nT /  Hz at 1 Hz Types of magnetic sensors

9 Superconducting Quantum Interference Device (SQUID) ILIL IRIR I Tot =I L +I R B A  I Tot Super-conductor Tunnel junctions

10 Flux-locked loop feedback IbIb preamp Integrator VOVO NSNS Shielded box in cryostat  I Tot 

11 SQUID measurement of quantum bits “Phase Qubit” Superconducting loop with junction IQIQ Sensor: asymmetric DC SQUID 1  0 Asymmetric current self biases SQUID Off On SQUID off - no interaction/dissipation operate qubit SQUID on – Measure qubit I Tot  ILIL

12 Measure quantum coherence IsIs I  wave 100  m Qubit DC SQUID Bias

13 SQUID magnetometer considerations Advantages High sensitivity with large pickup Arbitrary pickup loop geometry Magnetically “clean” Linear with feedback Disadvantages Lower sensitivity for small loops Cryogenic operation Bandwidth limited by feedback High power High cost

14 Nuclear Precession Magnetometers Proton magnetometer - H 2 O, Methanol, Kerosene Polarize protons in medium with high field Remove field - measure precession frequency Scalar technique Overhauser effect Polarize protons using electron spins Tempone – electron spin resonance pump Lower power Pumping frequency far from measurement He 3 – optical pumping of electrons => protons B ext M

15 Nuclear magnetometer applications Stable Predominant geological survey tool Medium – high sensitivity ~ 0.1 nT/ Spatial resolution ~ 10’s cm 2 Medium - high operation fields Medium power ~ W –Proton ~ 10 pT/  1Hz –Overhauser ~ 100 fT

16 Electron spin M Z optical magnetometers – He 4 C.P. light B ext (z) He 4 23S123S1 RF depopulates 1,-1 => 0, creates more absorbtion 1 0 => 2  signal on resonance I (%) RF frequency I

17 M Z magnetometer properties Higher frequency that proton nuclear resonance ~1000

18 M X magnetometers – optical pumping Alkali metals– Na 23,K 39, Rb 85,87 Heated cell Optical Microwave hf Energy D1 RF coils B ext Recent advances:  Spin exchange, relaxation-free K magnetometer  Chip scale atomic magnetometer (CSAM) 90 o

19 SERF M X magnetometers - K Optimal K-atom concentration (180 C) Very low field operation (<10  G) => No precession between K-K collisions Intrinsically vector capability Unshielded version: Lockin feedback on helmholtz - xzy ~ 1 pT/  Hz above 5 Hz I. M. Savukov, M. V. Romalis. PRA (2005). S. J. Seltzer and M. V. Romalis, APL (2004). 1 fT/  Hz

20 NIST CSAM Package 4.5 mm 1.7 mm Volume: 19 mm 3 Power Consumption: 198 mW P. D. D. Schwindt, et al. Appl. Phys. Lett. 90, (2007). Anticipated improvements to < 100 fT/  Hz, 20 mW Magnetic field noise [pT RMS / Hz 1/2 ] Frequency [Hz] 5.9 pT / Hz 1/2 over 1-10 Hz

21 Fluxgate Principle of Operation: Soft magnetic core M=  Modulate magnetization Flux “gated” when core saturated B-H curve shifts with applied field => Asymmetric V out Readout & linearize with feedback v out M H B ext Size ~ 10’s cm 3 H mod

22 Billingsley “best” fluxgate specifications Noise:  3.0 picoTesla 1Hz Range: ± 65  Tesla standard Accuracy: ±.02 % of Full Scale Zero offset:  +/- 5 nanoTesla Susceptibility To Perming: < ± 5 nanoTesla Shift with ± 5 Gauss applied Axial Alignment: Orthogonality better than ± 0.1° (0.02 ° special) Digital Output Resolution: 28 bits Conversion speed: 25 microseconds per sample Linearity: ±.001% of Full Scale Scale Factor Temperature Shift: .002 % / ° Celsius typical Power: 16 to milliWatts Field Measurement.Weight ~ 909 grams PVC housing Size w/Underwater Housing 7.8cm Diameter x 30.5 cm Length (PVC ) Price: ~1 k$

23 Typical fluxgate specifications Billingsley (2007) S~ 10 pT/  1 Hz Dynamic range – 65  T Field resolution

24 Noise sources –Thermal fluctuation of magnetization –Incoherent rotation of magnetization during switching –1/f magnetization jumps of two-state systems –Electronics noise Recent Advances  Radial magnetization => smoother rotation  Micro-fluxgate fabrication

25 GMI

26 Magnetoelectric

27 AMR

28 GMR

29 TMR

30 Hybrid GMR/Superconductor

31 Anisotropic magneto-resistive sensor Honeywell HMC 1001/1002 Frequency (Hz) Noise level (pT/  Hz)

32 Measured magnetic field - B z Calculated currents Intel flip-chip RAM a with short

33 Magneto-Optic

34 2 nd derivative unshielded pickup coil Ambient laboratory environment < 1 Monolayer Fe resolution at 10 cm ~ 100 pT/  8 Hz noise floor